For one of fiber laser apparatuses for use in a processing machine, for example, an MO-PA (Master oscillator-Power Amplifier) fiber laser apparatus is known in which a light beam generated from a seed light source such as a laser oscillator (Master oscillator (MO)) is amplified at an amplifier (a Power Amplifier (PA)) including an amplification optical fiber and emitted. For such a fiber laser apparatus, a wavelength conversion element is used to convert a light beam having a wavelength range of near infrared light into a short wavelength side, and a light beam having a wavelength range of visible light is emitted.
In performing wavelength conversion like this, when a light beam before wavelength conversion includes a large number of modes, such a tendency is observed that it is not enabled to efficiently perform wavelength conversion. For this reason, it is desirable that only a light beam in a fundamental mode be included in an incident light beam to a wavelength conversion element as much as possible and a light beam in a higher mode be not included. On the other hand, in association with a high output fiber laser apparatus, it is desired to use a multimode fiber whose core diameter is greater than that of a single mode fiber for an optical fiber such as an amplification optical fiber in order to propagate a light beam of greater power. Even in this case, it is desired to emit a light beam of excellent beam quality that a light beam in a fundamental mode is included and a light beam in a higher mode is decreased.
Patent Document 1 below describes a fiber laser apparatus including an amplification double cladding fiber that can propagate a multimode light beam. In this fiber laser apparatus, a mode converter that excites only a light beam in an LP01 mode (a fundamental mode) so as to include only a light beam in the LP01 mode in an incident light beam to a core, and the light beam in the LP01 mode can be mainly amplified in an amplification double cladding fiber that propagates a light beam in a multimode.